1. Field of the Invention
This invention relates to magnetoresistive devices (MR) for detecting magnetic fields, and more particularly, to a sandwich structure incorporating ferromagnetic and antiferromagnetic layers.
2. Background Art
Large magnetoresistances or change in magnetoresistances are needed for read heads in future high density magnetic storage devices. Current magnetoresistive devices used in read heads of storage devices use Permalloy wherein the magnetoresistance only changes a few percent such as 5% or less. "Giant" magnetoresistances and multilayer structures such as 100 layers where each layer is thin, for example 10 .ANG., are a possible solution but require large magnetic fields such as 10-20 kOe to get a reduction of 80% in resistivity. The layers may alternate between Co and Cu.
Another magnetoresistive device is the "spin-valve" structure comprising a thin layer of Co, a thin layer of Cu and a thin layer of Co to provide a 10-15% change in resistivity as a function of the applied field. While the structure of the three layers is simple, i.e. Co/Cu/Co, the magnitude of the magnetoresistance is difficult to control.
In U.S. Pat. No. 5,014,147 by S.S.P. Parkin et al. entitled, "Magnetoresistive Sensor with Improved Antiferromagnetic Film", an antiferromagnetic layer is formed in direct contact with the magnetoresistive layer. The antiferromagnetic layer comprises Fe.sub.1-x Mn.sub.x where x is within the range from 0.3 to 0.4. The magnetoresistive layer comprises NiFe.
In U.S. Pat. No. 5,159,513 by B. Dieny et al. entitled, "Magnetoresistive Sensor Based on the Spin Valve Effect", a layered structure is described comprising a first and second thin film layer of ferromagnetic material formed on a substrate wherein the first and second thin film layers are separated by a thin film layer of non-magnetic metallic material. At least one of the layers of ferromagnetic material comprises cobalt or cobalt alloys. The magnetization direction of the first layer of ferromagnetic material is substantially perpendicular to the magnetization direction of the second layer of ferromagnetic material at zero applied magnetic field.
In U.S. Pat. No. 4,949,039 which issued on Aug. 14, 1990 to P. Grunberg et al. entitled, "Magnetic Field Sensor with Ferromagnetic Thin Layers having Magnetically Antiparallel Polarized Components", a magnetic-field sensor is described having a first ferromagnetic layer magnetized in a first direction, an intermediate layer of non-ferromagnetic material forming an interface with the first layer, and a second ferromagnetic layer forming an interface with the intermediate layer and magnetically polarized with one magnetization-direction component in a direction opposite to the first direction so that the first and second layers are polarized with one component magnetically antiparallel to the first direction. The intermediate layer is composed of a material which causes a spin dependent electron scattering at the interfaces with the ferromagnetic layers. The intermediate layer has a thickness less than the mean free path length of conductivity electrons in the intermediate layer.
In a publication by G. Binasch et al., Physical Review B., V. 39, p. 4828 (1989) entitled, "Enhanced Magnetoresistance in Layered Magnetic Structure with Antiferromagnetic Interlayer Exchange", a layered magnetic structure was described which yields enhanced magnetoresistance effects by antiparallel alignment of the magnetization.
In a publication by W. H. Meiklejohn and C. P. Bean, Physical Review, V. 105, p. 904 (1957) entitled, "New Magnetic Anisotropy", a new type of anisotropy was discovered and described as exchange anisotropy. The anisotropy is the result of an interaction between the spins of cobalt atoms in a ferromagnetic material and the cobalt ions in a antiferromagnetic cobalt oxide. The material was fine particles of cobalt having a diameter in the range from 100-1000 .ANG.. It had a cobaltous oxide coating. Exchange anisotropy has also been described between Ni and NiO. In a publication by A. E. Berkowitz and J. H. Greiner, J. Appl. Phys., V. 36, 3330 (1965) entitled, "Exchange Anisotropy and Strain Interactions in the Ni-NiO System", the exchange anisotropy coupling and the interaction of the field Ni and NiO spin systems were examined for Ni films on single-crystal NiO substrates. The films of Ni and NiO constitute a ferromagnetic-antiferromagnetic combination.
Exchange anisotropies has also been observed in films of Fe-FeS. The exchange anisotropy was described in a publication by J. H. Greiner, J. Appl. Phys., V. 37, 1474 (1966) entitled, "Exchange Anisotropy Properties in Sulfided Iron Films". A layer of antiferromagnetic FeS of 100 .ANG. on a layer of ferromagnetic Fe of 1000 .ANG. produced a 7 Oe shift of the B-H loop. The exchange interaction between the spins across the interface was considered analogous to that in the Co-CoO system described by W. H. Meiklejohn and C. P. Bean, Phys. Rev. 102, 1413 (1956); 105, 904 (1957).
In a publication by J. H. Greiner, IBM Technical Disclosure Bulletin, V. 8, p. 1420 (1966) entitled, "Films with Shifted B-H Loops", exchange coupling was described between ferromagnetic iron and antiferromagnetic ferous sulfide. Similar properties were also observed in alloy films containing iron such as nickel-iron and cobalt-iron films after subjecting the film to hydrogen sulfide at an elevated temperature to provide a sulfide layer of the alloy.